This invention relates to a digital subscriber line transmission method, apparatus and system in which a subscriber line (referred to as a “metallic line” below) connecting a subscriber exchange and a subscriber terminal is utilized as a high-speed data communication line. More particularly, the invention relates to a digital subscriber line transmission method, apparatus and system, which is based upon ISDN ping-pong transmission or TDD-xDSL transmission, used in a periodic crosstalk noise environment.
Multimedia services such as the Internet have become widespread throughout society inclusive of the ordinary home and there is increasing demand for early provision of economical, highly reliable digital subscriber line transmission systems and apparatus for utilizing these services.
xDSL Technology
xDSL (Digital Subscriber Line) is known as a technique that provides a digital subscriber line transmission system in which an existing telephone line is utilized as a high-speed data communication line. xSDL is a transmission scheme that utilizes a telephone line and is one modulation/demodulation technique. xDSL is broadly divided into symmetric xDSL and asymmetric xDSL. In symmetric xDSL, the upstream transmission rate from the subscriber residence (referred to as the “subscriber side” below) to the accommodating office (referred to as the “office side” below) and the downstream transmission rate from the office side to the subscriber side are symmetrical; in asymmetric xDSL, the upstream and downstream transmission rates are asymmetrical.
A typical example of asymmetric xDSL is ADSL (Asymmetric DSL) and typical examples of symmetric xDSL are HDSL (High-bit-rate DSL) and SHDSL (Single-pair High-bit-rate DSL). VDSL (Very high-bit-rate DSL) is available as an xDSL technique that is capable of being utilized as both an asymmetric and symmetric DSL. A modulation scheme such as DMT (Discrete Multitone) or CAP (Carrierless Amplitude Phase modulation), etc., is being used for each xDSL system. Examples of ITU-T recommendations concerning ADSL are G.dmt and G.lite, in which the downstream transmission rates are on the order of 6 Mbps and 1.5 Mbps, respectively. Both of these adopt DMT modulation as the modulation method.
DMT Modulation
DMT modulation will be described taking G.dmt as an example. The description will relate only to modulation/demodulation in the downstream direction from the office side to the subscriber side.
With DMT modulation, as shown in FIG. 29, a frequency band of 1.104 MHz is divided into M(=255)−number of multicarriers #1˜#255 at intervals of Δf (=4.3125 KHz). In training carried out before communication, the S/N ratios of the respective carriers #1˜#255 are measured and it is decided, depending upon the S/N ratios, with which modulation method among 4-QAM, 16-QAM, 64-QAM, 128-QAM . . . modulation methods data is to be transmitted in each carrier. For example, 4-QAM is assigned to a carrier having a small S/N ratio and 16-QAM, 64-QAM, 128-QAM . . . are assigned successively as the S/N ratio increases. It should be noted that 4-QAM is a modulation scheme in which two bits are transmitted at a time, 16-QAM a modulation scheme in which four bits are transmitted at a time, 64-QAM a modulation scheme in which six bits are transmitted at a time, and 128-QAM a modulation scheme in which seven bits are transmitted at a time. Among schemes in which signals are transmitted simultaneously in upstream and downstream directions, a frequency-division transmission scheme uses carriers #1˜#32 of the 255 carriers for the upstream direction from the subscriber side to the office side, and uses carriers #33˜#255 for the downstream direction from the office side to the subscriber side. With a scheme in which signals are not transmitted simultaneously in the upstream and downstream directions, it is readily feasible to use all of the carriers #1˜#255 for the upstream and downstream directions.
FIG. 30 is a functional block diagram of a subscriber line transmission system based upon DMT modulation. Entered transmission data addressed to a subscriber is stored in an amount conforming to the time for one symbol (= 1/4000 sec) in a serial-parallel conversion buffer (Serial-to-Parallel Buffer) 10. The stored data is divided into transmission bit counts per carrier decided and saved in a transmit bitmap 60 by training in advance. The divided data is then input to an encoder 20. More specifically, since the QAM modulation scheme of each carrier is known from training, one symbol's worth of a bit sequence is divided bk bits at a time, where the bit count bk conforms to the QAM modulation scheme of each carrier, and the bits are input to the encoder 20. As a result, the total number of output bits per symbol is Σbk (k=1−M). The encoder 20 converts each carrier corresponding to an input bit sequence to signal-point data (signal-point data on a constellation diagram) for performing quadrature amplitude modulation (QAM) and inputs the converted data to an Inverse Fast-Fourier Transform (IFFT) unit 30. The IFFT unit 30 applies quadrature amplitude modulation to each signal point by performing an IFFT operation and inputs the processed data to a parallel-to-serial conversion buffer (Parallel-to-Serial Buffer) 40. Here a total 32 samples, namely IFFT output samples 480-511, are duplicately attached to the beginning of a DMT signal as a cyclic prefix (the details of which will be described later). The parallel-to-serial conversion buffer 40 inputs 512+32 items of sample data to a D/A converter 50 successively in serial fashion. The D/A converter 50 converts the input digital data to an analog signal at a sampling frequency of 2.208 MHz and sends the analog signal to the subscriber side via a metallic line 70.
On the subscriber side, an A/D converter 80 converts the input analog signal to a 2.208-MHz digital signal and inputs the digital signal to a time domain equalizer (TEQ) 90. The latter applies processing to the input digital data in such a manner that inter-symbol interference (ISI) will fall within the cyclic prefix of 32 symbols, and inputs the processed data to a serial-to-parallel conversion buffer 100. The latter stores one DMT symbol's worth of data and subsequently removes the cyclic prefix and inputs one DMT symbol's worth of data to a fast-Fourier transform (FFT) unit 110 simultaneously in parallel fashion. The FFT unit 110 implements a fast-Fourier transform and generates (demodulates) 255 signal points. A frequency domain equalizer (FEQ) 120 subjects the demodulated 255 items of signal-point data to inter-channel interference (ICI) compensation. A decoder 130 decodes the 255 items of signal-point data in accordance with a receive bitmap 150, which has values identical with those of the transmit bitmap 60, and stores the data obtained by decoding in a parallel-to-serial conversion buffer 140. The data is subsequently read out of this buffer in the form of a bit serial. This data constitutes the receive data.
Crosstalk from ISDN ping-pong transmission
ISDN time compression multiplexing (TCM) which is referred to as ISDN ping-pong transmission separates transmit and receive intervals in time-shared fashion (the total of one transmit interval and one receive interval is 2.5 ms) and makes the transmit and receive timings the same for all neighboring devices. With ISDN ping-pong transmission, 2B+D 144 kbps transmit data is demarcated every 2.5 ms, compressed to 320 kbps by rate conversion and transmitted in the transmit interval. As a consequence, the frequency band of ISDN ping-pong transmission overlaps the frequency band of ADSL (or of G.dmt), as shown in FIG. 31. Already existing telephone lines have a design optimized to a frequency band of about 200 Hz -3.4 kHz, which is the frequency band of the human voice. If ADSL and ISDN high-frequency signals are passed through such telephone lines, the fact that the lines are bundled together as shown in FIG. 32 allows the ISDN signal to leak into the ADSL telephone line and act as noise that interferes with the ADSL communication. Such noise is crosstalk noise. The ADSL transmission rate is limited by the level of this crosstalk noise.
FIG. 33 A and FIG. 33 B are a diagrams useful in describing interference (crosstalk) from an ISDN line to an ADSL, in which FIG. 33A is for describing interference on an ADSL unit on the office side (ATU-C: ADSL Transceiver Unit at the Central office end) and FIG. 33B is for describing interference on an ADSL unit on the subscriber side (ATU-R: ADSL Transceiver Unit at the Central terminal end). In FIG. 33A, the ADSL unit ATU-C on the office side is influenced by a large amount of noise when an OCU (Office Channel Unit) of the ISDN line is transmitting. This crosstalk noise is referred to as near-end crosstalk noise (NEXT). On the other hand, when a DSU (Digital Service Unit) is transmitting, this signal acts as noise by leaking into the ATU-C. This crosstalk noise is referred to as far-end crosstalk noise (FEXT). FEXT is remote with regard to the ATU-C, i.e., is noise from the remote end. In comparison with NEXT, FEXT has a fairly low level.
In FIG. 33 B, the ADSL unit ATU-R on the subscriber side is influenced by a large amount of noise when a DSU of the ISDN line is transmitting. This crosstalk noise is referred to as near-end crosstalk noise (NEXT). On the other hand, when an OCU is transmitting, this signal acts as far-end crosstalk noise (FEXT) by leaking into the ATU-R. In comparison with NEXT, FEXT has a fairly low level. Thus, in ADSL communication, it is necessary to reduce the effects of NEXT.
As mentioned above, when an ISDN ping-pong transmission line is close to an ADSL, the ADSL is influenced by TCM crosstalk (Time compression multiplexing crosstalk) from the ISDN ping-pong transmission line in a manner set forth below. In accordance with ISDN ping-pong transmission, the office side transmits the downstream data in the first-half cycles of 400 Hz in sync with an ISDN 400-Hz signal TTR which is TCM-ISDN timing reference as shown in FIG. 34, and the subscriber side transmits the upstream data in the second-half cycles after it receives the downstream data. The ADSL unit ATU-C on the office side, therefore, is influenced by near-end crosstalk (NEXT1) from the ISDN in the first-half cycles of 400 Hz and is influenced by far-end crosstalk (FEXT1) from the upstream data of the ISDN on the subscriber side in the second-half cycles.
In a manner converse with respect to the office side, the ADSL unit ATU-R on the subscriber side is influenced by FEXT2 in the first-half cycles of 400 Hz and is influenced by NEXT2 in the second-half cycles. The time intervals in which the effects of NEXT and FEXT are received shall be referred to as NEXT and FEXT intervals, respectively. FIG. 34 illustrates the NEXT and FEXT intervals on the subscriber side.
Sliding-Window Method
A “sliding-window method” has been proposed in the specification of Japanese Patent Application 10-144913 (issuing as Japanese Patent No. 3,480,313) for the purpose of providing a digital subscriber line transmission system that is capable of transmitting an ADSL signal satisfactorily in an environment where there is the above-described crosstalk from an ISDN ping-pong transmission. The sliding-window method is such that in the case of the downstream direction in which an ADSL signal is transmitted from an ADSL unit (ATU-C) on the office side to an ADSL unit (ATU-R) on the subscriber side, the state of the ADSL signal transmitted by the ADSL unit (ATU-C) on the office side in an environment where there is crosstalk from an ISDN ping-pong transmission is decided as set forth below. The method includes dual bitmap and FEXT bitmap methods.
Specifically, if an ADSL symbol (DMT symbol) SB to be transmitted falls entirely within the FEXT interval on the subscriber side, as shown in FIG. 34, the ADSL unit (ATU-C) on the office side transmits this symbol as an inside symbol ISB by a sliding window SLW through a high-density transmission. If a symbol SB to be transmitted falls even partially within the NEXT interval on the subscriber side, the ADSL unit (ATU-C) on the office side transmits this symbol as an outside symbol OSB through a low-density transmission (dual bitmap method). In the upstream direction also, the ADSL unit (ATU-R) on the subscriber side transmits the ADSL symbols upon dividing them into inside and outside symbols ISB, OSB, respectively, through a method similar to that employed in the downstream direction.
With the dual bitmap method, symbols are transmitted in low density even outside the sliding window SLW in the downstream direction. However, there is also the FEXT bitmap method, in which the ADSL unit (ATU-C) on the office side transmits only a pilot-tone signal PLT, which is a tone-signal for timing synchronization, outside the sliding window SLW. In this case the ADSL unit (ATU-R) on the subscriber side makes no transmission outside the sliding window SLW in the upstream direction.
FIG. 35 is a diagram showing the relationship between transmit/receive in the OCU of the ISDN and the ADSL symbols in the ADSL unit ATU-C on the office side, this FIG. 35 illustrates ADSL symbols in both the dual bitmap and FEXT bitmap cases.
Bitmap Creation
To support the dual bitmap method, it is required that two types of bitmaps, namely a bitmap for the inside symbols and a bitmap for the outside symbols, be prepared at the time of training in both the transmit bitmap unit 60 and receive bitmap unit 150 shown in FIG. 30. With the FEXT bitmap method, the outside-symbol bitmap of these two types of bitmaps is unnecessary.
The bitmap which shows the bit count assigned to each carrier is decided on the receiving side. That is, the number of assigned bits for upstream signals is decided on the office side and the number of assigned bits for downstream signals is decided on the subscriber side. When training is performed, the ADSL units on the office and subscriber side decide the bitmaps in accordance with a protocol referred to as “B & G (bit & gain)”.
FIG. 36 is a diagram useful in describing the B & G protocol for the upstream direction. (1) After the ADSL units recognize each other at the time of training, the ADSL unit ATU-R on the subscriber side sends several frequency signals to the opposing ADSL unit ATU-C on the office side, by way of example. (2) The ADSL unit ATU-C on the office side measures the noise level and received-signal level on a per-carrier basis and calculates the S/N ratios. (3) The ADSL unit ATU-C on the office side creates a bitmap based upon the S/N ratios calculated and reports this bitmap and transmission level to the ADSL unit ATU-R on the subscriber side. (4) The ADSL unit ATU-R on the subscriber side performs DMT modulation based upon the reported bitmap and transmission-level information and transmits the resulting data.
FIG. 37 is a block diagram of an arrangement in which S/N ratio is measured by the ADSL unit ATU-R on the subscriber side. Received data enters a demodulator 210, which outputs signal-point data on a per-carrier basis as demodulated data. Further, a reference 220 outputs carrier-by-carrier signal-point data that should be received originally. A difference ERROR develops between the signal-point data from the reference and the demodulated signal-point data, and the ERROR for each carrier is input to a selector 260.
An internal clock 230 of the device is frequency-divided to 400 Hz by a frequency divider 240 and the resulting signal is input to a phase discriminator 250. The 400-Hz signal has its phase matched beforehand to that of a 400-Hz signal (ISDN 400-Hz signal) on the office side by 400-Hz information transmitted from the office side via the demodulator 210. Using the 400-Hz signal input thereto, the phase discriminator 250 determines whether a received DMT symbol lies within the FEXT interval, the NEXT interval or outside these intervals and inputs the result to the selector 260. The latter outputs the above-mentioned ERROR signal to a NEXT-interval S/N measurement unit 270 or FEXT-interval S/N measurement unit 280 in accordance with the information that enters from the phase discriminator 250. Each S/N measurement unit integrates ERROR to calculate the S/N ratio and each outputs the S/N ratio to a transmit-bit-count conversion unit 290 carrier by carrier. From the entered carrier-by-carrier S/N ratios, the transmit-bit-count conversion unit 290 calculates the bit count (bitmap) transmitted carrier by carrier and calculates a bitmap b-NEXT for the NEXT interval and a bitmap b-FEXT for the FEXT interval.
Frame Structure
A hyperframe has been introduced for the purpose of providing a digital subscriber line transmission system that is capable of transmitting an ADSL signal satisfactorily in an environment where there is the above-described crosstalk from an ISDN ping-pong transmission. The ISDN ping-pong transmission switches between transmit/receive every half-cycle of the 400-Hz clock whose period is 2.5 ms. On the other hand, one symbol, which is the unit of transmission in ADSL transmission whose institutionalization as a global standard is proceeding, has a duration of about 0.246 ms. Accordingly, since 34 cycles of the ISDN ping-pong transmission, which is the least common multiple of the two types of communication, and the length of time of 345 DMT symbols in ADSL transmission coincide, this interval is defined as a “hyperframe”.
As shown in FIG. 38, it is so arranged that one frame becomes one symbol in ADSL. At the time of normal data communication, one superframe is constructed from 68 ADSL frames for data and one synchronization frame (S). There are also cases where an inverse synchronization symbol (I) is used instead of the synchronization symbol (S). The inverse synchronization symbol (I) is a symbol realized by rotating the phase of each carrier of the synchronization symbol (S) by 180°. As shown in FIG. 38, one hyperframe is constructed by collecting together five superframes (=345 symbols). FIG. 38 illustrates the case for the downstream direction in which the ADSL unit ATU-C on the office side transmits an ADSL signal to the ADSL unit ATU-R on the subscriber side. In this case it has been decided that the inverse synchronization symbol (I) be situated in the fourth superframe of one hyperframe. In the case of the upstream direction, the inverse synchronization symbol (I) is contained in the first superframe of one hyperframe. Further, one hyperframe is synchronized to 34 cycles of the 400-Hz signal in ISDN ping-pong transmission.
Alternative Frame Structure
In a case where an ISDN ping-pong transmission line is close to an ADSL, the ADSL is affected by both NEXT and FEXT TCM crosstalk from the ISDN ping-pong transmission line, as set forth above. For the purpose of providing a digital subscriber line transmission system that is capable of transmitting an ADSL signal satisfactorily in an environment where there is the above-described crosstalk from an ISDN ping-pong transmission, there is a method available which, unlike that which relies upon the above-mentioned hyperframe, transmits ADSL symbols upon synchronizing them to an ISDN ping-pong transmission.
In accordance with ISDN ping-pong transmission, the OCU on the office side transmits the downstream data in the first-half cycles of 400 Hz and receives the upstream data in the second-half cycles of 400 Hz in sync with the ISDN 400-Hz signal TTR, as illustrated in FIG. 39. In ADSL transmission also, the ADSL unit on the office side transmits ADSL symbols for the downstream FEXT interval in the first-half cycles of 400 Hz and transmits ADSL symbols for the downstream NEXT interval in the second-half cycles of 400 Hz in sync with the ISDN 400-Hz signal TTR. This is true also with regard-to the ADSL unit on the subscriber side. That is, two bitmaps are prepared, namely-a bitmap (DMT symbol A) for the NEXT reception interval and a bitmap (DMT symbol B) for the FEXT reception interval. Then, as shown in FIG. 39, the number of transmitted bits is reduced to improve S/N tolerance by transmitting the DMT symbols A in the NEXT interval and the number of transmitted bits is increased to enlarge transmission capacity by transmitting the DMT symbols B in the FEXT interval. By setting the cyclic prefix length to an appropriate length at this time, the number of ADSL symbols for the FEXT interval and the number of ADSL symbols for the NEXT interval are made to coincide. For example, a cyclic prefix of 40 samples which results in 250 μs per DMT symbol is adopted as opposed to an original cyclic prefix of 32 samples which results in 246 μs per DMT symbol, whereby one period of TCM crosstalk and the length of time of ten DMT signals are made to agree.
Introduction of TDD-xDSL
A TDD-xDSL (time-division duplex-xDSL) scheme is being considered as an xDSL scheme that does not use the above-described sliding window and hyperframes. The TDD-xDSL scheme is one which transmits symbols in sync with the above-mentioned ISDN ping-pong transmission but, unlike the method described above, it does not transmit TDD-xDSL symbols in the NEXT intervals. That is, the TDD-xDSL scheme uses an xDSL in the upstream and downstream directions in time-shared fashion and employs all 255 carriers #1-#255 in data transmission in the upstream and downstream directions.
When a TDD-xDSL symbol sequence 460 is transmitted in sync with an ISDN ping-pong transmission on the office side, a TDD-xDSL symbol sequence 480 received on the subscriber side is affected only by FEXT 440 from the ISDN, as shown in FIG. 40. When a TDD-xDSL symbol sequence 490 is transmitted in sync with an ISDN ping-pong transmission on the subscriber side, a TDD-xDSL symbol sequence 470 received on the office side is affected only by FEXT 430 from the ISDN. Accordingly, it is possible for a TDD-xDSL symbol sequence to avoid being affected by NEXT from an ISDN ping-pong transmission. In accordance with this transmission system, the two types of bitmaps that were required with the dual bitmap method are no longer necessary; here only one is sufficient, just as in the FEXT bitmap method.
ISI Removal Method
The time domain equalizer (TEQ) shown in FIG. 30 operates using the cyclic prefix in the manner described below.
A DMT symbol which enters the parallel-to-serial conversion buffer 40 in FIG. 30 represents a signal state that has no waveform distortion, as illustrated in (a) of FIG. 41. The parallel-to-serial conversion buffer 40 executes processing through which the 32 symbols at the end of this DMT symbol are added onto the beginning of the DMT symbol by copying, as illustrated in (b) of FIG. 41. The portion added on is referred to as the cyclic prefix. The DMT symbol onto which the cyclic prefix has been added is transmitted to the receiving side following subsequent processing on the transmitting side, as shown in (c) of FIG. 41.
A signal that has been received via the metallic line 70, the amplitude and delay characteristics of which are not constant with respect to frequency, becomes distorted owing to the influence of inter-symbol interference (ISI), as illustrated in (d) of FIG. 41.
However, the TEQ 90 has its constants set by training in such a manner that ISI falls within the cyclic prefix of 32 symbols (this is referred to as “TEQ training”). When the TEQ 90 receives the signal indicated at (d) in FIG. 41, therefore, the TEQ 90 executes processing in such a manner that ISI will fall within the cyclic prefix of 32 symbols, as shown in (e) of FIG. 41. Thereafter, the serial-to-parallel buffer 100 removes the cyclic prefix from the TEQ output. As a result, it is possible to obtain a DMT symbol from which the effects of ISI have been eliminated, as shown in (f) in FIG. 41.
Influence of ISI on xDSL Symbol
The influence of ISI on the xDSL symbol will be described with reference to FIG. 42. Illustrated in (a) of FIG. 42 is an ADSL transmit symbol sequence in a case where a continuous signal is transmitted at the time of training. It is assumed here that there is no continuity between the shaded ADSL transmit symbol illustrated in (a) of FIG. 42 and the ADSL transmit signal that precedes it. Illustrated in (b) of FIG. 42(b) is an ADSL receive symbol sequence corresponding to the ADSL transmit symbol sequence in (a) of FIG. 42 before TEQ training, and illustrated in (c) of FIG. 42 is an ADSL receive symbol sequence corresponding to the ADSL transmit symbol sequence in (a) of FIG. 42 after TEQ training.
Further, Illustrated in (d) of FIG. 42 is an ADSL transmit symbol sequence onto which a cyclic prefix which prevails at the time of normal data communication has been added, and illustrated in (e) of FIG. 42 is an ADSL receive symbol sequence corresponding to the ADSL transmit symbol sequence in (d) of FIG. 42.
(1) Influence After TEQ Training
As set forth above, the TEQ acts to remove the effects of ISI from a receive signal by using the cyclic prefix. If the cyclic prefix is added onto each ADSL transmit training symbol at the time of normal data communication, as shown in (d) of FIG. 42, the TEQ executes processing in such a manner that ISI falls within only the cyclic prefix of 32 symbols, thereby removing the effects of ISI from the receive signal.
However, at the time of training in which a continuous signal based upon the same pattern is transmitted, a cyclic prefix is not added onto any ADSL transmit training symbol, as shown in (a) of FIG. 42. The reason for this is that since a continuous signal is not affected by ISI, so cyclic prefix is unnecessary for the training symbols. On the contrary, if a cyclic prefix were added on, the symbol rate would decline correspondingly and it is therefore better not to add on a cyclic prefix.
However, in a case where a burst symbol sequence is sent as a transmit signal, as in the sliding-window method (FEXT bitmap method) or in the method (TDD-xDSL) of transmitting symbols in sync with ISDN ping-pong transmission, continuity of the transmit signal is lost. As a result, at the time of training in which a continuous signal is transmitted, the ADSL receive symbol at the beginning of the ADSL receive symbol sequence is influenced by waveform distortion corresponding to ISI, as shown in (c) of FIG. 42, even when TEQ training has been completed, and training cannot be carried out using this ADSL receive symbol at this beginning of the sequence.
(2) Influence Before TEQ Training
Illustrated in (b) of FIG. 42 is an ADSL receive symbol sequence resulting from the ADSL transmit symbol sequence in (a) of FIG. 42 before TEQ training is performed. Here also, however, the ADSL receive symbol is distorted owing to the effects of ISI for the reason set forth above. In (c) of FIG. 42, the influence of waveform distortion corresponding to ISI on the leading ADSL receive symbol falls within 32 symbols owing to the TEQ. In (b) of FIG. 42, on the other hand, this is an ADSL receive symbol sequence before TEQ training is performed and in general, therefore, the influence of ISI on the ADSL receive symbol does not fall within 32 symbols. It is considered that before TEQ training is performed, the influence of waveform distortion acts upon ADSL receive symbols from the second onward as well, as shown in (b) of FIG. 42. Further, there are instances where the ADSL receive symbol at the end of the ADSL receive symbol sequence also is affected by ISI, though this is not illustrated in FIG. 42.
Thus, at the time of training in which a continuous signal based upon the same pattern is transmitted, a cyclic prefix is not added onto each transmit symbol. As a consequence, in TDD-xDSL transmission in which a burst symbol sequence is transmitted as a training signal, the receiving side cannot respond immediately at the rising edge of the burst symbol sequence and waveform distortion occurs at the beginning of the burst symbol sequence. Accordingly, training is carried out only by the remaining TDD-xDSL receive symbols that have not been influenced by waveform distortion. However, in an instance where four DMT symbols are transmitted in one burst of training, the DMT symbols capable of being used in training are three in number. A problem that arises, therefore, is prolonged training time.
Further, if the transmit training symbol sequence falls within the receive interval (NEXT interval) of an ISDN ping-pong transmission in TDD-xDSL transmission, the transmission is influenced by NEXT from the ISDN line and the TDD-xDSL transmission cannot be performed with a favorable S/N ratio.
Further, in TDD-xDSL transmission, there is no established technique for setting the frequency of a pilot-tone signal used as a timing regeneration signal so as to assure continuity of sample data in contiguous transmit burst signal sequences. A problem that arises is that processing cannot be executed at an accurate timing.
Further, with TDD-xDSL, there is a phase difference between the phase of a training symbol onto which a cyclic prefix for transceiver training has not been added and the phase of a symbol obtained by removing a cyclic prefix from a symbol onto which this cyclic prefix was added at the time of normal data communication. As a consequence, a problem which arises is that the phase of a timing regeneration signal (pilot-tone signal) shifts when a sequence makes a transition from a training symbol onto which a cyclic prefix has not be added to a symbol onto which a cyclic prefix has been added (a training→normal communication transition).
Furthermore, though it is necessary for the office side to synchronize a TDD-xDSL transmission to an ISDN ping-pong transmission, the same is true on the subscriber side as well. Though the office side can use an 8 kHz-network clock to obtain a 400-Hz synchronization signal to which the ISDN ping-pong transmission is synchronized, the subscriber side cannot obtain this 400-Hz synchronization signal. Accordingly, it is important for the subscriber side to have the office side report the transmission phase of the TDD-xDSL accurately so that this information can be obtained. This makes it necessary to provide means for reporting the transmission phase from the office side to the subscriber side efficiently.
The foregoing is for a case where crosstalk from an ISDN line to an xDSL is taken into consideration. However, crosstalk is not limited to that from an ISDN line; there is crosstalk also from other xDSLs within the same cable. In particular, since TDD-xDSL transmission is synchronized to the 400-Hz siqnal TTR of an ISDN ping-pong transmission to perform downstream and upstream transmissions alternately in time-shared fashion, as mentioned above, the xDSL is influenced by crosstalk (NEXT, FEXT), which is similar to that of ISDN ping-pong transmission, from other TDD-xDSLs. Accordingly, the above-described problem holds not only for crosstalk from an ISDN line but also for crosstalk from other TDD-xDSLs.